Devices using spatial filtering schemes are well known in the prior art for measuring the speed of automobiles and other vehicles without physical contact. An example of an optical system used in such a device is described in Japanese Patent Publication, Kokai 52-143081. In this device, the light reflected by the object whose speed is to be measured is conducted through a lens to a detector in a spatial filter system. A panel is furnished between the lens and the detector, and the panel has a small aperture at the focal point of the lens.
To minimize the influence of stray light such as sunlight, street lights or tunnel illumination in such optical measurement devices, it is common to use a scheme employing light whose intensity has been modulated by a photoelectric switch, especially high-speed modulated light.
However, for its photodetector, this type of existing optical system uses a comb toothtype photodiode to serve the function of a spatial filter. This type of photodiode has a large surface area. Accordingly, the response speed of the light receiving device is slow. For this reason, such a photodiode responds poorly to modulation imposed on the sources at the sites of emission. Another problem is the relatively high cost of the required photodetectors.
One effective way to improve the response time of the photodetector is to combine a purely optical spatial filter with a high-speed photodetector. One example of a device using a purely optical spatial filter can be found in Japanese Patent Publication 58-2662, which describes a speedometer employing a prism-type spatial filter. The spatial filter in this speedometer consists of a laminated prism. The light from the object whose speed is to be measured enters this prism. However, in this device the light is widely diffused when it exits the prism making it difficult to channel it efficiently to the photodetectors. Thus, the efficiency with which the light from the sources can be utilized is poor.
Another example of an existing spatial filter using a prism array is the filter described in Japanese Patent Publication 62-133164. This type of spatial filter uses a prism array constructed by stacking a number of prisms or columnar lenses, and is thus laborious to manufacture with acceptable precision. This filter also suffers a loss in the quantity of light received due to the scattering of light caused by reflections off the refracting surfaces. Furthermore, the ratio of the period of the aperture to its width is not restricted, so it inevitably becomes 2:1. In contrast, the ratio of transmitting to non-transmitting components in an aperture restriction device is 1:1. The light pattern develops components at the third and higher order harmonics of the spatial frequency causing an unfavorable SN ratio.
An existing speed sensor which uses polarized light is described in Japanese Patent Publication 63-190957, as part of a scheme to prevent mistakes in detection when the object in question is submerged. However, with this type of sensor it is necessary to have the light strike the surface of the water at the Brewster angle so as to minimize reflective loss. For this reason the light source and the receptor unit must be kept well apart, which results in a relatively large measurement device.
In addition to these problems relating to the prior art devices, the optical systems used in existing speed sensors would be difficult to install in an optical system of a compact sensor device.